1. Field of the Invention
The present invention relates to an optical module.
2. Description of the Related Art
A CAN-type optical module used in optical communication generally includes an electrically-grounded stem and a lead terminal that passes through the stem and is insulated from the stem. The stem and a cap mounted to the stem form a housing for accommodating an optical semiconductor device. The lead terminal and the stem forma coaxial line. One end portion of the lead terminal is connected to the optical semiconductor device. Another end portion of the lead terminal is connected to a drive device configured to output a modulated electrical signal, via a wiring substrate, e.g., an FPC having a signal line and a ground formed along the signal line.
Increases in communication speed are leading to an increase in the frequency of the modulated electrical signal output from the drive device. Due to the increase in the frequency of the electrical signal, reflection of the electrical signal tends to occur in a transmission line at locations at which there are mismatches in the characteristic impedance. Characteristic impedance mismatches tend to occur between, for example, the coaxial line, which is formed by the lead terminal and the stem, and the wiring substrate, e.g., an FPC. At locations in the transmission line at which there are mismatches in the characteristic impedance, reflection waves of the electrical signals are generated to interfere with original modulated electrical signals, resulting in lowered waveform quality of optical signals. In order to suppress to a minimum the effects of characteristic impedance mismatches on the optical waveform at a connecting portion between the lead terminal and the FPC at which characteristic impedance mismatches tend to occur, in Japanese Patent Application Laid-open No. 2009-302438, there are proposed structures for suppressing characteristic impedance mismatches.
Specifically, in Japanese Patent Application Laid-open No. 2009-302438, a flexible printed board, which is vertically mounted on a CAN-type package in the related art, is horizontally connected to the CAN-type package, thereby suppressing impedance mismatches.
In recent years, optical modules have been strongly demanded to achieve not only a lowered cost but also an increased speed. If high-speed optical signals in the 25 Gbit/s class can be transmitted with inexpensive CAN-type optical modules, both the demands for a lowered cost and an increased communication speed can be satisfied. However, if a unique flexible printed board connection method, e.g., the one proposed in Japanese Patent Application Laid-open No. 2009-302438, is employed for suppressing impedance mismatches, soldering is complicated, thereby influencing processing costs and yields of optical modules. As a result, the optical modules may not be stably manufactured and supplied.